1. Field of the Invention
The present invention relates to a gain variable amplifier for use in a communication device such as a portable telephone and, more particularly, to a gain variable amplifier with improved linearity of a power gain change.
2. Description of Related Art
In order to realize higher speech quality, a portable telephone in recent years is required to have the function of accurately controlling the strength of waves of transmission in accordance with the strength of waves received. In a gain variable amplifier used for each of transmitting and receiving units, it is therefore necessary to realize linearity of a power gain change over a wide wave strength range.
In a conventional gain variable amplifier, the linearity of a power gain change realized by an amplifier of one stage is limited. Consequently, as shown in FIG. 1A, amplifiers of multi-stages (two amplifiers Amp1 and Amp2 in FIG. 1A) are connected and gains of the amplifiers are added, thereby assuring the linearity of the power gain change in a wide range as a whole.
In another conventional technique, as an attempt to widen the linearity of the power gain change in the amplifier of one stage, as shown in an operating principle diagram of FIG. 2, in order to control the gain in a differential amplification circuit 10 in a gain variable amplifier 500, a response of a gain control signal VAGC to a control input signal VCNT of a gain control circuit 50 is controlled so as to be either linear or to have a monotone increase characteristic of which curve opens upwards in a linear coordinate system. FIG. 3 shows a specific example of such a circuit. In the circuit of FIG. 3, the gain control circuit 50 also has a differential input circuit configuration. An input voltage VCLS obtained by shifting the level of the control input signal VCNT so as to be linear by a level shifting circuit 51 is supplied to a differential pair in the gain control circuit 50. A specific operation is as shown in FIG. 4. The level of the input voltage VCLS obtained by the level shifting of the level shifting circuit 51 is shifted so that the input output characteristic of the gain control circuit 50 expresses a curve which opens upwards or is linear, thereby outputting the gain control signal VAGC having a response characteristic peculiar to the control input signal VCNT. By matching the characteristic with that of the differential amplification circuit 10, a linear gain change characteristic is realized.
Specifically, when the input voltage VCLS shifts from VA to VB with respect to the control input signal VCNT, an output current ICNT of the gain control circuit 50 has a monotone increase characteristic of which curve opens upwards. At the time of converting the output current ICNT to a voltage by a load resistor and outputting the voltage as the gain control signal VAGC, the gain control signal VAGC is adjusted so as to match the linear characteristic of the differential amplification circuit 10. Consequently, a monotone increase characteristic of which curve opens upwards can be obtained as an output characteristic of the differential amplification circuit 10. When the characteristic is expressed in decibels as the power gain, a linear gain change characteristic can be obtained in response to the control input signal VCNT.
When the input voltage VCLS is shifted from VB to VC with respect to the control input signal VCNT, the output current ICNT characteristic of the gain control circuit 50 becomes linear. When the characteristic is adjusted so as to match the monotone increase characteristic of which curve opens upwards of the differential amplification circuit 10, a monotone increase characteristic of which curve opens upwards is derived as an output characteristic of the differential amplification circuit 10. Thus, a linear gain change characteristic can be similarly obtained.
In the conventional techniques, the technique of connecting amplifiers of multi-stages has a problem that, since the number of component circuits is large, a number of components are necessary so that circuit layout is rigidly limited. Since current consumption is high, there is also a problem such that continuous use time of a portable device such as a portable telephone is limited, and it is also difficult to realize high density packing due to heat generated in association with current consumption.
In the another conventional technique in which the gain control is adjusted to attain the linear characteristic or the monotone increase characteristic of which curve opens upwards, only a limited region of the input output characteristics of the differential amplification circuit 10 is used. When the differential amplification circuit 10 is used beyond the region, the linearity of the gain change is lost and the circuit cannot be used. The region in the input output characteristics of the differential amplification circuit 10 which can realize the linearity is therefore limited and the linearity over a wide range cannot be assured. There is also a problem such that, since the differential amplification circuit is used within the narrow operating range of the gain control signal VAGC, it is difficult to assure sufficient operating accuracy.
The invention has been achieved to solve the problems of the conventional techniques and its object is to provide a gain variable amplifier of one stage capable of realizing linearity of a power gain change over a wide range by properly changing a gain control characteristic in response to a control input signal VCNT.
In order to achieve the object, according to one aspect of the invention, there is provided a gain variable amplifier comprising: a gain control circuit having a gain control characteristic of outputting a predetermined gain control signal in accordance with a control input signal; and a differential amplification circuit having an input output characteristic of varying a gain by switching a differential output current passing through a differential pair by the gain control signal, wherein the gain control circuit variably controls the gain control characteristic in accordance with the input output characteristic.
Consequently, the gain control circuit can properly vary the gain control characteristic of the gain control signal in response to the control input signal in accordance with the input output characteristic of the differential amplification circuit. A predetermined gain change characteristic can be therefore given to the control input signal and the linear gain change over a wide input output range of the differential amplification circuit can be realized.
According to another aspect of the invention, in the gain variable amplifier, when the gain control signal is supplied to the differential amplification circuit and is in a first region in which the input output characteristic is a linear characteristic in a linear coordinate system, the gain control circuit variably controls the gain control characteristic so as to be a first monotone increase control characteristic of which curve opens upwards in a linear coordinate system, when the gain control signal is supplied to the differential amplification circuit and is in a second region in which the input output characteristic is a monotonous increase characteristic of which curve opens downwards in a linear coordinate system, the gain control circuit variably controls the gain control characteristic to a second monotone increase control characteristic of which curve is sharper than that of the first monotone increase control characteristic, and when the gain control signal is supplied to the differential amplification circuit and is in a third region in which the input output characteristic is a monotone increase characteristic of which curve opens upwards in a linear coordinate system, the gain control circuit variably controls the gain control characteristic to be a linear control characteristic.
In the first region in which the input output characteristic of the differential amplification circuit is linear, the gain control circuit variably controls the gain control characteristic so as to be a first monotone increase control characteristic of which curve opens upwards in the linear coordinate system. In the second region in which the input output characteristic of the differential amplification circuit is a monotone increase characteristic of which curve opens downwards in the linear coordinate system, the gain control circuit variably controls the gain control characteristic to a second monotone increase control characteristic of which curve is sharper than that of the first monotone increase control characteristic. In the third region in which the input output characteristic of the differential amplification circuit is a monotone increase characteristic of which curve opens upwards in the linear coordinate system, the gain control circuit can variably control the gain control characteristic to be a linear control characteristic. Since the gain control characteristic can be optimally set over the entire range of the input output characteristic of the differential amplification circuit, the linearity of the gain change can be maintained over the entire region and the linearity over a wide range can be assured.
According to further another aspect of the invention, in the gain variable amplifier, the gain control circuit can have a transconductance amplifier construction, and variably control the gain control characteristic by varying a conductance characteristic of the transconductance amplifier construction.
By varying the conductance characteristic, a current output of the transconductance amplifier construction can be freely set in response to the control input signal, the current can be converted to a voltage, and the voltage can be used as a gain control signal. Consequently, the optimum gain control characteristic can be easily and certainly obtained over a wide range in the input output characteristic of the differential amplification circuit, and the linearity of the gain change over a wide range can be assured.
According to further another aspect of the invention, in the gain variable amplifier, the transconductance amplifier construction can have a composite differential input circuit of multiple inputs, and an individual control input signal having a predetermined inclination and a predetermined offset value can be supplied in response to each input signal supplied to the composite differential input circuit.
By supplying the individual control input signal which has a predetermined inclination and a predetermined offset value and which changes linearly to each of inputs of the composite differential input circuit of multiple inputs, the operation of the composite differential input circuit can be switched according to the output current response of the composite differential input circuit according to the predetermined inclination of the individual control input signal and the predetermined offset value by a sum of the output currents of differential pairs of the composite differential input circuit. Therefore, the input output characteristics of the differential pairs of the composite differential input circuit can be freely combined by a simple linearity change in response to the individual input signal, the optimum gain control characteristic is easily obtained over a wide range in the input output characteristic of the differential amplification circuit, and the linearity of the gain change over a wide range can be assured.
According to further another aspect of the invention, in the gain variable amplifier, the gain control circuit comprises: a first transistor and a second transistor constructing a differential pair; a first diode whose anode is connected to an emitter of the first transistor; a second diode whose anode is connected to an emitter of the second transistor; a third transistor; a first differential input circuit in which a cathode of the first diode and a cathode of the second diode are connected to each other, a reference voltage is applied to a base of the first transistor, and a first individual control input signal is supplied to a base of the second transistor; a second differential input circuit in which an emitter of the third transistor is connected to the cathodes of the first and second diodes, the cathodes being connected to each other, a second individual control input signal is supplied to a base of the third transistor, the circuit being achieved with the first transistor; the composite differential input circuit being constructed by the first and second differential input circuits; a fourth transistor; and a resistor connected to an emitter of the fourth transistor. A current source circuit of a predetermined conductance is formed by connecting the other end of the resistor to a reference potential and supplying a third individual control input signal to a base of the fourth transistor, and a sum of an output current of the first differential input circuit, an output current of the second differential input circuit and an output current of the current source circuit is converted to a voltage and the voltage is outputted as the gain control signal.
By supplying the first individual control input signal to the base of the second transistor, the output current of the first differential input circuit has the first monotone increase control characteristic of which curve opens upwards in the linear coordinate system. By supplying the second individual control input signal to the base of the third transistor, the output current of the second differential input circuit has the second monotone increase control characteristic of which curve is sharper than that of the first monotone increase control characteristic. By supplying the third individual control input signal to the base of the fourth transistor, the output current of the current source circuit constructed by the fourth transistor and the resistor connected to the emitter has the linear control characteristic. By the above setting, as the sum of the output currents having the control characteristics by adjusting the inclination of the individual control input signals and the offset values, the gain control characteristic can be switched from the linear control characteristic via the first monotone increase control characteristic to the second monotone increase control. By the combination of the input output characteristics of the differential amplification circuit, the linearity of the gain change can be maintained over the entire range and the linearity over a wide range can be assured.
In the first differential input circuit, the first and second diodes are connected in the forward direction to the emitters of the first and second transistors constructing the differential pair. Consequently, the operating point of the differential pair can be set higher only by an amount corresponding to the forward voltage of the diodes, and the first individual control input signal to be supplied to the base of the second transistor can be set to a high voltage level. Thus, the operating range of the first individual control input signal can be set to a very accurate voltage range for the signal generating circuit, and the linearity of the gain change characteristic can be realized with higher accuracy.
The circuit constructions can be easily realized by using bipolar transistors.
According to further another aspect of the invention, in the gain variable amplifier, the gain control circuit comprises: a first transistor and a second transistor which form a differential pair; a first diode whose anode is connected to a source of the first transistor; a second diode whose anode is connected to a source of the second transistor; a third transistor; a first differential input circuit in which a cathode of the first diode and a cathode of the second diode are connected to each other, a reference voltage is applied to a gate of the first transistor, and a first individual control input signal is supplied to a gate of the second transistor; a second differential input circuit in which a source of the third transistor is connected to the cathodes of the first and second diodes, the cathodes being connected to each other, a second individual control input signal is supplied to a gate of the third transistor, the circuit being achieved with the first transistor; and the composite differential input circuit being constructed by the first and second differential input circuits. A sum of an output current of the first differential input circuit and an output current of the second differential input circuit is converted to a voltage and the voltage is outputted as the gain control signal.
By supplying the first individual control input signal to the gate of the second transistor, the output current of the first differential input circuit is set to have the first monotone increase control characteristic of which curve opens upwards in the linear coordinate system. By supplying the second individual control input signal to the gate of the third transistor, the output current of the second differential input circuit is set to have the second monotone increase control characteristic of which curve is sharper than that of the first monotone increase control characteristic. Further, when it is set to use the linear region of the first or second differential input circuit as a linear control characteristic, as a sum of the output currents having the control characteristics by adjusting the inclination of each of the individual control input signals and the offset value, the gain control characteristic can be switched from the linear control characteristic via the first monotone increase control characteristic to the second monotone increase control characteristic. The linearity of the gain change can be maintained over the entire region by the combination with the input output characteristic of the differential amplification circuit, and the linearity over a wide range can be assured.
In the first differential input circuit, the first and second diodes are connected in the forward direction to the sources of the first and second transistors constructing the differential pair. Consequently, the operating point of the differential pair can be set higher only by an amount corresponding to the forward voltage of the diodes, and the first individual control input signal to be supplied to the gate of the second transistor can be set to a high voltage level. Thus, the operating range of the first individual control input signal can be set to a very accurate voltage range for the signal generating circuit, and the linearity of the gain change characteristic can be realized with higher accuracy.
The circuit constructions can be easily realized by using MOS transistors.
According to further another aspect of the invention, the gain variable amplifier further comprises a level shifting circuit. The individual control input signal having a predetermined inclination and a predetermined offset value can be generated by shifting the level of the control input signal.
Since the gain variable amplifier is provided with the level shifting circuit, the control input signal can be subjected to the level shifting to thereby obtain an individual control input signal having a predetermined inclination and a predetermined offset value. Thus, the linearity over a wider range can be assured by a single control input signal.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are purpose of illustration only and not intended as a definition of the limits of the invention.